Films comprising a mixture of polyester and polyether block copolymers are described in the prior art, as for example in WO 01/85451. WO 01/84451 discloses a mixture for making cards, said mixture comprising an amorphous copolyester and a thermoplastic polyester elastomer such as Hytrel® in amounts of more than 20% and less than 50% wt thermoplastic polyester elastomer.
In U.S. Pat. No. 6,436,531 each polymer is present in amounts of about 20 to about 80% wt based on the total weight of the blend, more preferably in an amount of at least 40% wt. A composition comprising 40% wt of Polyethylene Terephtalate (PET) and 60% wt of Hytrel®, based on the total weight of the blend. However in U.S. Pat. No. 6,436,531, the film is not biaxially stretched.
Biaxially stretched films are known in the art. U.S. Pat. No. 5,846,642 discloses a polyester film comprising a polyester mixed with a polyester based elastomer such as Hytrel® or Arnitel® at a ratio of 81:19 to 98:2 laminated with another polyester. This cast polyester film can be simultaneously or sequentially biaxially stretched.
JP 2002003707 discloses a biaxially oriented polyester film having an impact resistance in addition to linearly tearable properties without damaging gas-barrier properties. The film is obtained by dispersing a polyether-ester elastomer in the proportion of 5-30% wt in a PET and is characterised in that the elastomer contains 21-60% wt of polytetramethylene glycol (PTMG).
U.S. Pat. No. 5,912,060 discloses a mixture of thermoplastic polyester resins such as PET and polyester block copolymer as polyether ester block copolymer, in respective amounts of 50 to 95% wt and 50 to 5% wt. The film obtained can be biaxially stretched. The polyester block polymer of this reference can be a polyether; the sole disclosure is in a blend with 20% of the polyether. At such concentrations, the final film is not breathable. U.S. Pat. No. 6,458,437 discloses heat-shrinkable polyester films produced from polyester compositions containing polyesters at 50-95% wt of said polyester composition and polyester elastomers at 3-50% wt of said composition. The dicarboxylic acid components constituting the polyesters used may include terephthalic acid and the diol components constituting the polyesters used may include ethylene glycol. Polyester elastomers refer to polyester block copolymers composed of soft and hard segments, wherein the soft segments may be polyether ester segments, i.e. polyester elastomers can be polyether block copolymers. These films can be biaxially stretched. EP 644 226 discloses an easily tearable film made of two or more thermoplastic resins having an islands-sea structure comprising a continuous and a dispersed phase. The film may be biaxially stretched. Among others, Polyethylene terephthalate and polyester elastomers as Hytrel® are cited. The proportions of the different phase is so that the continuous phase/dispersed phase equals about 35-98/2-65, preferably about 50-95/5-50% wt. A compatibilizing agent may be added to the combination of the thermoplastic resins in order to make the dispersed particles finer. However the amount of this compatibilizing agent should not exceed 30 parts by weight relative to 100 parts by weight of the total weight of the thermoplastic resins composing the continuous and the dispersed phase otherwise the islands-sea structure may disappear.
Breathable films are known in the art. US 2002/0076554 discloses polyether block copolymers producing inherently breathable films, such as Pebax® and Hytrel®. This patent application also discloses films which are biaxially stretched to become breathable films. In this patent application, breathability is imparted to the biaxially stretched film by using fillers in the film polymer formulation, extruding the filler/matrix polymer formulation into a film and then stretching the film sufficiently to create voids around the particles, thereby making the film breathable.
U.S. Pat. No. 6,582,810 also discloses such a breathable and elastic film based on an extruded filler/matrix polymer formulation stretched to impart breathability. In this patent, the matrix polymer formulation which may constitute about 35 to 95% by weight of the breathable film can be a polyamide polyether block copolymer, and the filler particles may be polyethylene terephthalate. Multi-block elastomeric copolymers, such as Pebax® polyether block amide, Hytrel® polyester can be used as elastic fibers to provide elasticity. According to U.S. Pat. No. 6,582,810, when the film is desired to have superior breathability and moderate strength, the breathable film may include about 30-55% wt of the matrix polymer, and 45-70% wt of the particulate filler.
US 2002/098353 discloses various blends of polymers. Examples 21-24 of said documents disclose blends with 50% or 60% of a polyether polymer such as Hytrel together with a polyester. The film is merely cast and is not stretched. The film in this document is used for the manufacture of tapes that will withstand high temperatures. There is no mention of the breathability of the films, and no mention of any end-use that may require said property.
Method for drawing plastic films simultaneously in both longitudinal and lateral directions is also known in the art. Indeed, U.S. Pat. No. 3,890,421 discloses such a method; its content is incorporated therein by reference.
Although some of the foregoing films known in the art may be either breathable or biaxially stretched, none of them is a breathable film that is bi-axially stretched. Breathable film are useful in many applications as they exhibit special gas selective properties and liquid permeation properties. Biaxially stretched films are sought for as biaxially stretching improves some properties of plastic films, such as resistance, elasticity and reduces the thickness of the films. The problem faced when trying to sequentially bi-axially stretch the breathable film known in the art is that the film breaks or shrinks. There is thus a need for a breathable film which can be biaxially stretched, i.e. a composition which enables the production of such a film together with a method of manufacturing such a film, in order to improve the mechanical and the thermal properties of breathable films.